Generally, particulate matters in diesel engine exhaust are composed of carbonaceous soot particulates with average diameter of 0.3 μm, comprising soots, sulfates and heavy hydrocarbons, which are derived from lubricating oil and unburned fuel. Especially, soot particulates, caused by incomplete combustion of diesel fuel, increase visual discomfort and the degree of air pollution that causes offensive odors, and as well, can damage human health. Therefore, emission control standards for soot particulates in diesel engine exhaust have been further strengthened, but practically applicable technologies satisfying the current emission control standards have not been presently commercially developed. Thus, it is urgently required to develop novel technologies for abatement of the generation of soot particulates and after-treatment of the soot particulates. At present, there are proposed methods of reducing emissions of soot particulates by improvement of engine control technologies or by use of a fuel additive, and methods of removing soot particulates from exhaust gas through a post-treatment device, such as a diesel particulate filter.
As for the method of reducing emissions of the soot particulates, desirable results cannot be obtained by the improvement of engine technology alone for some vehicles. Meanwhile, the use of fuel additives is not cost-effective and may cause secondary pollution by emission thereof into the atmosphere.
The method using post-treatment device includes a trapping process of soot particulates in the exhaust gas by use of a filter medium, and a combustion process of the trapped soot particulates to regenerate inherent filtering performance of the filter medium. However, use of the post-treatment device suffers from excessive increase of back- pressure due to the accumulation of the trapped soots when the trapped soots are not continuously regenerated, resulting in worsening engine performance.
Hence, a number of different options for controlling particulates are diversely attempted, and are largely classified into active regeneration and passive catalytic regeneration.
The active regeneration is used to forcibly combust the soot particulates to regenerate the filter by use of a burner or an electric heater, which is advantageous in terms of superior regeneration performance. However, the above process is disadvantageous in that excessive thermal shock applied to the filter medium leads to the damage of filter medium, while negating economic benefits due to the use of complicated control units.
Additionally, the regeneration process of the filter medium by adding an organic metal additive to diesel fuel is disadvantageous in that the added metal component is deposited in the engine, and fine metallic additive particles not trapped in the filter medium is discharged to the atmosphere, thus causing secondary pollution.
Regarding the passive catalytic regeneration, U.S. Pat. No. 4,902,487 (corresponding to Japanese Patent No. 3,012,249) discloses a catalytic, two-stage, passive particulate filter system including a monolithic oxidation catalyst (upstream) and a ceramic wall-flow diesel filter (downstream). The monolithic oxidation catalyst allows to convert a portion of nitrogen monoxide (NO) contained in diesel exhaust gases into nitrogen dioxide (NO2), which is a much stronger oxidizing agent than oxygen. As such, NO2 has a concentration range of 100-2,000 ppm, and reacts with the soot particulates trapped on the particulate filter downstream, and thus, allows the sooty particulates to be combusted at 225-300° C., thereby purifying the diesel engine exhaust gases. However, since the oxidation catalyst upstream acts also to facilitate generation of sulfate by oxidation of sulfur dioxide (SO2) in the exhaust gas, the use of ultra low sulfur diesel (ULSD) fuel is required. In addition, NO2 formation rate rapidly decreases at higher temperatures due to the constraint of thermodynamic equilibrium. Therefore, it is required to develop a catalytic particulate filter that has a sufficiently low balance point temperature (BPT) to continuously remove soot particulates at the typical exhaust temperature range.